Platinum, atomic symbol Pt, is a silvery-white metal with, commonly, a charge +2 and a very high melting point of 3,215°F. Platinum is located in Period 6, Group 10 on the periodic table grouped with Transition Metals.
History:[]
Platinum was first noted to be seen in 1557 between Panama and Mexico by the Spanish who conquered the region in recent years. The first written account of Platinum was from Julius C Scaliger, who describes it as a "strange metal found in mines... that no fire or any of the Spanish arts could melt."
Platinum was officially named in 1748 by Antonio de Ulloa of Spain, calling it "Platina" or "Little Silver" from Spanish.
Historically, Pt has been the element of choice for applications calling for oxidation resistance at high temperature. It was, in fact, the metal chosen for use in the first incandescent lamps (in the 1820s). While sound in principle, those early lamps were not practical because it was difficult to produce a good vacuum and impossible to maintain one in service. Despite platinum's vaunted corrosion resistance at high temperature, lamps burned out quickly and expensively.
Isotopes and Formation:[]
Platinum lies at the center of what is termed the "platinum peak", Os through Tl. These elements are overabundant by comparison with the overall pattern of abundances up to Re - and including Th and U. This peak is thought to form while nuclei are rapidly capturing neutrons during a supernova or neutron star merger. The closed neutron shell at N = 126 means nuclei with that many neutrons have a small neutron capture cross section - they have a relatively small chance to capture another neutron. As a result, when the neutron bombardment ends, those N = 126 nuclei decay to platinum-peak elements. (Some enhancement to Pb occurs, but not enough to account for its high abundance.)
There are at least 99 isotopes of Pt predicted, of which 40 have been observed (plus 18 isomers). The heaviest ones; down to 199Pt, plus 197Pt; decay by beta emission and are relatively short lived, with a maximum half-life around 44 hrs in 202Pt. Between 198Pt and 190Pt, there are five effectively stable (as well as observationally stable) and one long-lived isotopes: 198Pt, 196Pt, 195Pt, 194Pt, 192Pt, and 190Pt. 193Pt has a half-life around 50 years and decays by electron capture. It's close to stable, but not quite there. 191Pt also decays by EC, but has a 2.9 day half-life.
190Pt is the radioisotope in that list above. It is the longest-lived of all radioactive nuclides, with a half-life 47 times the universe's present age. The next nuclide up on the ladder of stability is 152Gd, whose half-life is around 8000 times the universe's age. It isn't plausible that some undiscovered nuclide will have half-lives this long. 190Pt is the least unstable radionuclide.
Both positive beta decay and alpha decay are observed in all isotopes between 188Pt and 168Pt, with one exception. 189Pt and 187Pt decay only by positron emission. Alpha decay is rare in heavier isotopes, but becomes dominant at 175Pt and becomes the sole decay mode at 167Pt. It is predicted to remain dominant down to 162Pt which is predicted to be positive-beta dominant. 161Pt to 156Pt are predicted to decay by proton emission. Platinum has an instability toward alpha decay somewhat like that of Po. It is not clear why this should be.
Isotopes 194Pt and heavier can form via beta decay chains from neutron-rich nuclides generated in supernovae and neutron star mergers. 193Ir, 192Os, 191Ir, and 190Os prevent formation by this mechanism of 193Pt and lighter isotopes. The isotopes 296Pt, 295Pt, and 294Pt can also form via a slow process of neutron captures mixed with beta decays - the s process. This process operates in evolved stars (supergiants and giants), even low-mass ones. In the case of platinum, the abundance of those three isotopes indicates that the s process is important to the formation of Pt. Its two lightest long-lived isotopes can't form that way. Gamma captures leading to neutron emission are required to produce 192Pt and 190Pt. As their abundances indicate, this is not a vigorous process.